MSan: disallow uninitialized data in GLES2Implementation::BufferDataHelper.

MSan doesn't undestand shared memory, so a buffer that has been tainted with
uninitialized values would still be considered uninitialized even after a write
on the other end (the GPU). This results in confusing uninit reports in the
renderer after readback. Doing an explicit check early should make those reports
less confusing and less flaky. We might even find some real bugs!

BUG=445745
R=zmo@chromium.org

Review URL: https://codereview.chromium.org/867903004

Cr-Commit-Position: refs/heads/master@{#314098}

gpu/command_buffer/client/gles2_implementation.cc

@@ -17,6 +17,7 @@
 #include <sstream>
 #include <string>
 #include "base/bind.h"
+#include "base/compiler_specific.h"
 #include "base/numerics/safe_math.h"
 #include "gpu/command_buffer/client/buffer_tracker.h"
 #include "gpu/command_buffer/client/gpu_control.h"
@@ -1239,6 +1240,14 @@ void GLES2Implementation::BufferDataHelper(
   if (!ValidateSize("glBufferData", size))
     return;
 
+#if defined(MEMORY_SANITIZER) && !defined(OS_NACL)
+  // Do not upload uninitialized data. Even if it's not a bug, it can cause a
+  // bogus MSan report during a readback later. This is because MSan doesn't
+  // understand shared memory and would assume we were reading back the same
+  // unintialized data.
+  if (data) __msan_check_mem_is_initialized(data, size);
+#endif
+
   GLuint buffer_id;
   if (GetBoundPixelTransferBuffer(target, "glBufferData", &buffer_id)) {
     if (!buffer_id) {